achondrite,any stony meteorite containing no chondrules (small, roughly spherical objects that formed in the solar nebula). The only exclusions are carbonaceous chondrites of the CI group, which, though they are clearly chondrites, are so heavily altered by water that any evidence for their having contained chondrules is lost. Achondrites, constituting about 4 percent of the known meteorites, are similar in appearance to terrestrial igneous rocks that have a low silica contentscontent, such as basalts, peridotites, and pyroxenites. Most formed by various melting and crystallization processes in within asteroids. Most The majority of achondrites belong to one of the following groups: acapulcoites, angrites, aubrites, chassignites, diogenites, eucrites, howardites, lodranites, nakhlites, shergottites, and ureilites. The shergottites, nakhlites, and chassignites almost certainy certainly came from Mars. There are also In addition, a small group of achondrites are believed to be derived from the Moon.See alsochondrite.